1. Field of the Invention
The invention relates to a method of and a device for measuring the blade clearance between the moving-blade tips and the blade carrier of thermal turbomachines.
2. Discussion of Background
In thermal turbomachines, the gap losses between the rotating moving-blade tips of the rotor and the parts of the stator (blade carrier) which are opposite them have a great effect on the efficiency of the machine. The greater these gap losses, the lower the efficiency. Therefore, the aim is to keep the radial moving-blade clearance in turbines and compressors as small as possible during operation. On the other hand, however, contact or, in the extreme case, abrasion of the parts must be prevented, since that could cause generation of heat and severe damage to the machine. The blade tips wear until the abrasion is complete, hard contact often leading to an enlarged blade clearance.
In contrast, however, abrasive coatings are sometimes used in order to abrade excessively long blades at the tips with slight generation of heat. In this method, the blades cut into the "soft" surface until there is no longer any contact.
In order to achieve a small amount of radial clearance over the long term, the radial expansion of the stator and of the rotor must be accurately controlled by the design. Simultaneous and identical expansion of stator and rotor in both the transient and the stable state would be ideal. However, that is rarely the case in actual machines, for the rotor is usually more solid than the stator on account of the strength requirements. Since both the heat-transfer conditions and usually the material for both parts are the same, the rotor is therefore thermally substantially slower than the stator. The thermal expansions of rotor and stator differ in the way they vary over time, especially during start-up and stopping of the machine. The radial clearance in the stable state is determined by the smallest clearance in the transient state (pinch point). The blade clearance is at its smallest during hot start of the machine. However, the largest clearance in the transient state must always be smaller than a certain maximum value in order to avoid, for example, the pumping of the compressor or to prevent local overheating downstream of the turbine.
The performance of modern turbomachines is improving. This can be attributed in particular to improved blade-clearance control. The need to measure the actual blade clearance, especially the blade clearance in the transient state, is continually increasing.
The measurement of the blade clearance by means of a measuring needle which can be abraded has hitherto been the most widespread method. The needle is fitted into the stator (blade carrier) during the assembly of the machine and projects radially opposite the rotating blade tips into the flow duct. The length of the measuring needle and the location for accommodating the needle in the stator are measured and are thus known. During the operation of the turbomachine, the measuring needle wears on account of abrasion. The disadvantage with this prior art consists in the fact that only the smallest transient blade clearance during the time between the first start of the turbomachine and the shut-off and the removal of the measuring needle can be determined. The advantage of this method is the high accuracy.
In addition, modern methods for the "remote measuring" of the blade clearance are known, for example by means of X-radiation and electric sparks as well as the measurement of the electrical capacitance and inductance. Most of these methods, however, are costly and are difficult to handle at high temperatures. Furthermore, the space required limits the use of a plurality of measuring points. The measuring accuracy of these methods is restricted.